Etymology
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frequent (adj.)

mid-15c., "ample, profuse," from Old French frequent, or directly from Latin frequentem (nominative frequens) "often, regular, repeated; in great numbers, crowded, numerous, filled, full, populous," which is of uncertain origin. Watkins says probably from PIE *bhrekw- "to cram together," and compares Greek phrassein "to fence in," Latin farcire "to cram," But Beekes regards the connection to the Greek word as "quite uncertain." Meaning "common, usual" is from 1530s; that of "happening at short intervals, often recurring" is from c. 1600.

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frequent (v.)
late 15c., "visit or associate with," from Old French frequenter "attend frequently; assemble, gather together," from Latin frequentare "visit regularly; do frequently, repeat; assemble in throngs," from frequentem (see frequent (adj.)). Meaning "visit often" is from 1550s. Related: Frequented; frequenter; frequenting.
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frequently (adv.)
"often and at short intervals," 1530s, from frequent (adj.) + -ly (2).
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unfrequented (adj.)
1580s, from un- (1) "not" + past participle of frequent (v.).
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frequence (n.)
1530s, "an assembling in large numbers," from French fréquence, from Latin frequentia "an assembling in great numbers" (see frequent). From c. 1600 as "frequent occurrence."
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infrequent (adj.)
1530s, "little used" (now obsolete); 1610s, "not occurring often," from Latin infrequentem (nominative infrequens) "occurring seldom, unusual; not crowded, absent," from in- "not" (see in- (1)) + frequens "repeated, regular, constant, often" (see frequent). Related: Infrequently.
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frequentative (n.)

"verb which expresses repetition of action," 1520s, from French fréquentatif, from Late Latin frequentativus "serving to denote the repetition of an act," from Latin frequentat-, past-participle stem of frequentare "visit regularly; do frequently, repeat," from frequentem (see frequent (adj.)). Frequentive is considered incorrect, because -ive adjectives are normally formed on the Latin past participle.

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infrequency (n.)
1670s, fact of being infrequent," from Latin infrequentia "a small number, thinness, scantiness," abstract noun from infrequentem (nominative infrequens) "occurring seldom, unusual; not crowded, absent," from in- "not" (see in- (1)) + frequens (see frequent). Older in this sense is infrequence (1640s). Earlier infrequency was used in a now-obsolete sense of "state of being unfrequented" (c. 1600).
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frequency (n.)
1550s, "state of being crowded" (now obsolete); 1640s, "fact of occurring often;" from Latin frequentia "an assembling in great numbers, a crowding; crowd, multitude, throng," from frequentem (see frequent). Sense in physics, "rate of recurrence," especially of a vibration, is from 1831. In radio electronics, frequency modulation (1922, abbreviated F.M.) as a system of broadcasting is distinguished from amplitude modulation (or A.M.).
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os- 
frequent form of ob- before -c- and -t- in words from Latin.
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